Dear @apetukho ,
In the same post you mention it is also discussed that it is not possible to update an already existing tree within a file Is there a way to save dataframe to an already existing root file? - #6 by Karl007 .This hasn’t changed meanwhile. But as Enrico suggests in the post, you could create a new tree with the branch you were trying to attach and just use it as a friend tree via TTree::AddFriend later on.
Cheers,
Vincenzo